OYSTERS, CLAMS, AND OTHER MOLLUSKS 233 



through the gills into tubes and thence into the 

 cavity above. As the water passes through the gills, 

 the colorless blood of the oyster is aerated. Thus the 

 oyster uses the same organs for eating and breathing. 



In the early days of the American industry, oysters 

 were very plentiful in all of the bays and harbors 

 from Massachusetts southward along the Atlantic 

 coast. Oysters could be taken in large quantities from 

 the public waters. But as the demand for this delect- 

 able shell-fish increased, fishing on the public grounds 

 became so intensive that large areas became ex- 

 hausted. Many harbors became so polluted with trade 

 wastes that oysters could not live. 



When the value of oysters became sufficiently high 

 to warrant the expenditure of the necessary effort, 

 oyster culture in privately owned or controlled waters 

 became the general order of the day. If it were not 

 for the commercial growing of oysters in protected 

 waters, these shell-fish would be very scarce indeed. 

 Oysters are still taken in large quantities from many 

 public grounds, but as the years pass a larger and 

 larger proportion are grown by the oystermen. 



The first operation in the growing of oysters is 

 the planting of old shells, which act as spat collectors. 

 The oysters spawn during July and August; conse- 

 quently the oysterman must have his beds of shells 

 ready at that time. The shells must not be placed 

 too soon, else the shells become covered with dirt and 

 then the set will not stick to them. If the oysters do 

 not spawn soon after the shells have been deposited 

 on the ground, they are stirred up with dredges to 



